1. Field
The present invention relates generally to data communication, and more specifically to techniques for mitigating deleterious effects due to link imbalance in a wireless communication system (e.g., a CDMA system).
2. Background
Wireless communication systems are widely deployed to provide various services such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users, and may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access techniques. CDMA systems may provide certain advantages over other types of system, including increased system capacity.
To improve reliability, a terminal may concurrently communicate with multiple base stations via a process often referred to as soft handover. Soft handover is typically supported for certain services (e.g., voice), but is often not supported for packet data on the downlink. This is because additional transmit power would be required to support soft handover on the downlink. Moreover, packet data service can tolerate longer delays, which then allows for implementation of a retransmission scheme. For packet data transmission on the downlink, one of the base stations with which the terminal is in communication may be designated as a “serving” base station, and only this base station transmits packet data to the terminal. Data packets received in error by the terminal (i.e., erased packets) may be identified via feedback information sent to the base station, which can then retransmit these erased packets.
To maximize system capacity, on the uplink in a CDMA system, the transmit power of each terminal is controlled by a power control loop such that the signal-to-noise-and-interference ratio (SNR) of an uplink transmission, as received at the base station, is maintained at a target SNR. This target SNR is often referred to as the setpoint. While in soft handover, the uplink transmit power of each terminal is typically adjusted based on an “OR-of-the-Down” rule whereby the terminal decreases its transmit power if any base station requests a decrease.
In certain instances, the base station with the best uplink for the terminal is not the serving base station. This phenomenon, which is referred to as link imbalance, may have a detrimental impact on the performance of packet data transmission. In particular, if link imbalance exists, then the uplink transmit power of the terminal would be adjusted based on the received SNR at the base station with the best uplink. However, this base station is not the one transmitting packet data to the terminal and receiving feedback information from the terminal. If the link imbalance is sufficiently great, then the serving base station may not reliably receive the feedback information from the terminal for erased packets. Performance may then be severely impacted by the failure to retransmit these erased packets.
There is therefore a need in the art for techniques to mitigate deleterious effects due to link imbalance in a wireless communication system.